Power line protection equipment present in an electric power substation will typically include a plurality of protective relays, instrument transformers for converting the values of voltage and current on the line to levels suitable for use in the protective relays, and circuit breakers responsive to the protective relays, as well as other accompanying elements. The protective relays are used to interrupt AC (alternating current) power distributed from the substation, in response to a protective relay detecting a fault on the power lines. Power for the equipment in the substation, including the protective relays, is provided by a DC supply system, the source of which is a battery. A battery charger is also usually part of the supply system. The DC supply system also typically provides power for communication within the substation.
Most substation battery systems operate on either 24, 48, 125 or 250 volts DC, although it is common to have two different battery systems in a substation, with different voltage levels, e.g. one system may operate at 125 volts while the other operates at 48 volts DC. Typically, DC power systems within a substation are operated ungrounded. Grounding of any part of a DC supply system within such a substation would thus typically be unintentional, i.e. inadvertent. While such inadvertent DC grounds are all of concern, a single inadvertent ground typically will not cause a major DC supply system failure, although it may affect the operation of certain of the equipment within the substation, including possibly the protective relays and the shunt trip coils or close coils which operate the circuit breakers, in particular circumstances. A second DC ground in the DC supply system, on the opposing polarity supply bus, will certainly affect the operation of the equipment or blow the fuses in the system.
A typical DC power system within a substation will include long lengths of wire connections, including wire extending from the battery source in a control house to remote locations in the substation, such as to circuit breakers and system disconnect apparatus. These long wire lengths present significant opportunities for inadvertent DC grounds. Inadvertent grounds can occur for a number of different reasons, but typically are due to a breakdown in the wire insulation. The damage or breakdown can be due to aging, weather, poor connections or other causes. This can occur in any of the wire runs themselves, whether short or long, even within the control house, or at connections to relay coils or switches as well as connections to or within the protective relays.
The DC power system includes plus and minus polarity DC busses, as well as extensive equipment connections. DC grounds on the plus and minus busses can be detected by conventional DC monitoring systems which are typically located in the vicinity of the battery portion of the system. However, DC grounds in the remainder of the DC supply system are usually very difficult and time-consuming to locate. In a substation involving a large plurality of protective relays, transformers, circuit breakers and associated contacts and coils, an indication by the conventional monitoring system of a DC ground will result in an alert to a repair crew, which has the task, upon arrival, if the DC ground is not found to be on the plus or minus bus, of sequentially removing from the substation power circuit each protective relay or other piece of equipment so as to locate the DC ground.
This task is compounded by the fact that specialized personnel must approve the temporary disconnection of circuit breakers in the substation, because removal of the DC power removes the protective relay equipment and the power source required to operate a breaker during a fault. Further, DC ground indications tend to occur more frequently in inclement weather conditions, such as rain storms. The repair of such DC grounds requires the technicians and operators to work within an energized (operating) substation in the rain, which can often be disconcerting to the repair personnel, due to obvious hazards. It is important to locate the DC failure while the power system is wet, because when the system dries out, the DC ground may disappear when the water leaves the break in the wiring insulation (until the next rain storm).
Hence, it is quite desirable to identify the location of inadvertent DC grounds within an electric power substation with greater particularity than is presently possible.